1、Basic Hydrology (HYDR 310)(Topic 2),Precipitation processes, measurements, and variability (Part I),Dr. Xixi (Jack) Wang, P.E.,Atmospheric Moisture,Moisture or water vapor is the source of precipitation Water vapor varies with location and time as a function of temperature and source of supply Moist

2、ure is greatest near the ocean Moisture generally decreases with latitude, altitude, and distance inland from sea Warm air is less dense than cold air Warm air tends to expend, whereas, cold air tends to compress,Atmospheric Moisture,measure of water vapor,remark,Changes with the surrounding air pre

3、ssure; not commonly used,Not influenced by air pressure,Not influenced by air pressure,Commonly used,Commonly used,Atmospheric Moisture: Vapor Pressure,Definition: measures the amount of water vapor in the air,The ideal gas equation:,The air and vapor have identical V, R, and T, thus:,pvapor measure

4、s the amount of vapor,Atmospheric Moisture: Saturation Vapor pressure,Definition: the static pressure of water vapor when the vapor phase of water is in equilibrium with the liquid phase of water,Goff-Gratch equation (1957):,Atmospheric Moisture: Saturation Vapor pressure,Arden Buck Equation (1996):

5、,Atmospheric Moisture: Dew Point Temperature,Equation I:,Atmospheric Moisture: Dew Point Temperature,Equation II:,Precipitation Generation,Warm air rises due to the temperature gradient The warm air cools by the upper cold air The rain drops fall speed exceeds the rising rate of the air The rain dro

6、ps are large enough to descend through the unsaturated air below the cloud base without completely evaporating before reaching the ground There are four mechanisms that generate vertical air motion,Air Motion Mechanisms: Orographic Uplift,Lifting of moist air currents over mountain barriers Common i

7、n the west coast of U.S.,Air Motion Mechanisms: Orographic Uplift,Example questions: The water vapor is measured to have a temperature of 18C at the sea level. Answer the following questions: Determine the altitude at which the water vapor starts to condense if the dew point temperature is 12C. Dete

8、rmine the temperature of the water vapor at the altitude of 1,800 m. Determine the temperature of the water vapor at the sea level if the vapor would start to condense at 1,800 m altitude.,Air Motion Mechanisms: Orographic Uplift,DALR: dry adiabatic lapse rate = 1C/100m SALR: saturated adiabatic lap

9、se rate = 0.6C/100m DALR and SALR indicate the temperature decrease rate with altitude,Air Motion Mechanisms: Convective Uplift,Caused by the rise of warm air The unstable air rises and cools until it reaches the altitude where its temperature equals that of the environmental lapse rate (ELR) ELR va

10、ries and is a point on the line of SALR Usually create thunderstorms,Air Motion Mechanisms: Convective Uplift,Air Motion Mechanisms: Convective Uplift,Air Motion Mechanisms: Cyclonic Uplift,Caused by unequal heating of the earths surface Air masses move from high-pressure regions to low-pressure regions Frontal precipitation results from the lifting of a warmer air mass over the frontal surface of a colder, denser air mass Nonfrontal precipitation results from that air is lifted as a result of horizontal convergence of the inflow